Presently, InGaAlN system (group III nitride) devices used as light sources of the colors from purple to blue and those from blue to green are manufactured by growing crystals on mainly a sapphire substrate or a SiC substrate using a MO-CVD (metal-organic chemical vapor deposition) method, a MBE (molecular beam epitaxy) method, or the like. Examples of problems when sapphire or SiC is used as a substrate include increased crystal defects caused by a large thermal expansion coefficient difference and lattice constant difference between the substrate and the group III nitride. Therefore, device characteristics are degraded, resulting in disadvantages such as a difficulty of lengthening the service life of light-emitting devices or increasing the operating power.
Furthermore, since the sapphire substrate has insulation properties, it is impossible to take out an electrode from the substrate as in typical light emitting devices and necessary to take it out from the front surface of crystal-grown nitride semiconductors. As a result, the device area becomes large and high costs result. Furthermore, in the group III nitride semiconductor device manufactured on the sapphire substrate, it is difficult to separate a chip by cleavage and not easy to obtain a resonator end face necessary for laser diodes (LDs) by cleavage. Therefore, a dry etching technique is used to obtain the resonator end face, or the sapphire substrate is polished to a thickness equal to or smaller than 100 μm to obtain the resonator end face in the same manner as the cleavage. In this case also, it is difficult to obtain the resonator end face and separate the chip in a single process as in typical LDs, causing complicated processes and high costs.
In order to solve these problems, GaN substrates have been earnestly desired, and so Patent Documents 1 and 2 have proposed a method of forming a thick film on a GaAs substrate and a sapphire substrate using a HVPE (hydride vapor phase epitaxy) method and removing these substrates later.
Although the GaN independent substrate is obtained by these methods, different kinds of materials such as GaAs or sapphire are basically used to obtain the substrate. Therefore, high-density crystal defects remain in the substrate due to the thermal expansion coefficient difference and the lattice constant difference between the group III nitride and the substrate materials. Even if the defective density could be reduced, it would fall in a range only from 105 to 106 cm−2. It is necessary to further reduce the defective density to achieve high-performance (high power and long service life) semiconductor devices. Furthermore, in order to manufacture one piece of substrate of group III nitride crystal, it is necessary to provide at least one piece of GasAs substrate or sapphire substrate as a ground substrate and remove the same. Accordingly, manufacturing costs are high due to the necessity of growing the thick film of several hundred μm by the vapor phase epitaxy, undergoing the complicated processes, and providing the extra ground substrate.
On the other hand, Patent Document 3 discloses a method of encapsulating sodium azide (NaN3) and the metal Ga as materials in a stainless-steel reaction vessel (size of the vessel: 7.5 mm in inner diameter, 100 mm in length) in a nitrogen atmosphere and holding the reaction vessel at temperatures of 600 through 800° C. for 24 through 100 hours so as to grow GaN crystals. Patent Document 3 characterizes practical growth conditions such as a possibility of growing crystals at relatively low temperatures of 600 through 800° C. and a relatively low in-vessel pressure of about 100 kg/cm2 at most. However, this method has a problem in that the obtained crystals are small in size not to reach 1 mm.
The present inventors have keenly studied how to achieve high-quality group III nitride crystals by reacting a mixed melt consisting of an alkaline metal and a group III metal with a group V material containing nitrogen. The inventions related to this are disclosed in Patent Documents 4 through 36. This method is called a flux method.
The flux method exhibits a possibility of growing extremely high-quality group III nitride crystals. There have been made inventions that improve and devise a method of growing high-quality group III nitride crystals and an apparatus for growing the same to achieve an increase in size and higher quality of the crystals. Examples of present technical problems include a further increase in size of the crystals.
As an inhibition to increasing the crystals in size, the vaporization of flux has to be taken into consideration. An alkaline metal is mainly used as the flux. As the alkaline metal is vaporized from the mixed melt containing the group III metal and the alkaline metal, the amount ratio of the flux to the group III metal fluctuates. This results in a variation in the crystal quality and an inhibition to increasing the crystal size.
The present inventors have improved the vaporization of flux in Patent Documents 9, 18, and 26.
Patent Document 9 discloses that an alkaline metal is confined in the reaction vessel by controlling the temperature above the front surface of a mixed melt and devising the introduction direction of a nitrogen material gas. Patent Document 18 discloses that the vaporization of an alkaline metal is suppressed by controlling the pressure of gas in the reaction vessel and devising the shape of a cover of a mixed melt holding vessel. Patent Document 26 discloses that a reduced alkaline metal is replenished by introducing another alkaline metal from the outside.
Thus, it is possible to suppress the fluctuation in the amount ratio of the flux to the group III metal. As a result, it is possible to stably grow crystals and achieve the reduction of the variation in the crystal quality and the increase in the crystal size.
However, in the typical flux method in which the mixed melt consisting of an alkaline metal and a group III metal is reacted with the group V material containing nitrogen so as to obtain GaN crystals, it is difficult to prevent the vaporization of the alkaline metal from the mixed melt to the outside, and it is difficult to control the fluctuation in the molar ratio of the alkaline metal to the group III metal which causes inhibition to increasing the crystal size.